Some chlorine-containing organic compounds are known to be hazardous. In consequence a strong movement exists to limit the chlorine content of organic compounds. Thus, crankcase lubricant manufacturers in some countries specify that their finished lubricants must have less than 50 parts per million by weight (ppm) chlorine.
Conventional polyisobutenyl dispersants used in crankcase lubricants frequently contain chlorine. Chlorine is introduced into these compounds in two ways. First, the polymer backbone is made by cationic polymerisation of isobutene in the presence of a strong Lewis acid and a promoter, usually an organoaluminum compound. Then, a particularly efficient way to introduce a succinic group requires halogenating the polymer at an olefinic bond and subsequent maleation. In this process most of the chlorine is liberated as hydrogen chloride. A small amount of chlorine remains and is carried forward in the process to be present in the finished dispersant.
Crankcase lubricants made with these dispersants thus contain chlorine. When used at conventional levels, the lubricants contain chlorine at levels greater than the 50 ppm specification desired in many locations. Despite their chlorine content, continued use of these dispersants is desirable because known processes to make low chlorine or chlorine-free products are less flexible, use raw materials less efficiently, and are more likely to produce undesirable deposits in the reactor than the halogenation/maleation process. Additionally, plants to make chlorine-free polyisobutenyl succinic anhydride (PIBSA) are less numerous making the supply of chlorine-free PIBSA tight.
Lubricant formulations having a chlorine-containing dispersant and meeting the new low chlorine requirements must use less of the chlorine-containing dispersant while still achieving suitable performance. Thus non-conventional dispersants are required. Possible non-conventional dispersants include dispersants made from extremely low chlorine thermal polyisobutenyl succinic anhydride (see e.g. U.S. Pat. No. 5,356,552 and references cited therein) and viscosity modifiers that also function as dispersants.
Viscosity modifiers are materials added to crankcase lubricants to impart high and low temperature operability. Viscosity modifiers that have been post reacted to provide dispersancy are known as multi-functional viscosity modifiers or dispersant viscosity modifiers. Multigrade oils typically contain one or more viscosity modifiers. Thus, the viscosity modifier acts to increase viscosity at high temperature thereby providing more protection to the engine at high speeds, without unduly increasing viscosity at low temperatures which would otherwise making starting a cold engine difficult. High temperature performance is usually specified by kinematic viscosity (kV) at 100.degree. C. (ASTM D445), while low temperature performance is specified in terms of cold cranking simulator (CCS) viscosity (ASTM D5293, which is a revision of ASTM D2602).
Viscosity grades are defined by the SAE Classification system according to these two measurements. SAE J300 defines the limits for kinematic viscosity and CCS as follows:
______________________________________ SAE VISCOSITY GRADES SAE Maximum CCS kV kV viscosity Viscosity 100.degree. C. mm.sup.2 /s 100.degree. C. mm.sup.2 /s grade 10.sup.-3 Pa .multidot. s @ (.degree. C.) minimum maximum ______________________________________ 0 W 3250 (-30) 3.8 5 W 3500 (-25) 3.8 -- 10 W 3500 (-20) 4.1 -- 15 W 3500 (-15) 5.6 -- 20 W 4500 (-10) 5.6 -- 25 W 6000 (-5) 9.3 -- 20 -- 5.6 &lt;9.3 30 -- 9.3 &lt;12.5 40 -- 12.5 &lt;16.3 50 -- 16.3 &lt;21.9 60 -- 21.9 &lt;26.1 ______________________________________
Multigrade oils meet the requirements of both low temperature and high temperature performance, and are thus identified by reference to both relevant grades. For example, an SAE 5W-30 multigrade oil has viscosity characteristics hat satisfy both the 5W and the 30 viscosity grade requirements--i.e. a maximum CCS viscosity of 3500.times.10.sup.-3 Pa.s at -25.degree. C., a minimum kV100.degree. C. of 9.3 mm.sup.2 /s and a maximum kV100.degree. C. of &lt;12.5 mm.sup.2 /s. Viscosity modifiers comprise polymers having an Mn of at least 20,000. For ease of handling viscosity modifiers are usually employed as oil solutions of such polymers.
Use of multifunctional viscosity modifiers to provide dispersancy lost upon decreasing the amount of chlorine-containing dispersant is economically more attractive than use of other non-conventional dispersants. But multi-functional viscosity modifiers have been found to impact adversely high temperature deposits. This adverse effect is particularly evident in high temperature engine tests such as the PV 1431 Volkswagen Intercooled Turbo Diesel engine test and the Caterpillar 1G2 engine test.
At the same time, formulations having relatively low total sulphated ash are also desired. Extremely high levels of sulphated ash have been implicated in combustion chamber deposits. Thus, one automobile manufacturer recommends a maximum total ash level of 1.2 wt % as determined by ASTM D874.
Metal-containing or ash-forming detergents function both as detergents to reduce or remove deposits and as acid neutralisers. Detergents generally comprise a metal salt of an acidic organic compound, typically metal salts of sulfonates, phenates, sulfurized phenates, or salicylates. The salts may contain a substantially stoichiometric amount of the metal in which case they are usually described as normal or neutral salts, and would typically have a total base number or TBN (as measured by ASTM D2896) of from 0 to less than 150. Large amounts of a metal base may be included by reacting an excess of a metal compound such as an oxide or hydroxide with an acidic gas such as carbon dioxide. The resulting overbased detergent comprises micelles of organic salt surrounding cores of inorganic metal base (e.g. carbonate). Such overbased detergents may have a TBN of 150 or greater, and typically ranging from 250 to 450 or more.
While metal-containing detergents are useful to control high temperature deposits on the piston, the requirement for low sulphated ash exacerbates the problems associated with use of multi-functional viscosity modifier in lieu of conventional dispersant because it limits the formulator's ability to use larger amounts of metal-containing detergents.
Accordingly a need exists for a lubricant formulated with a chlorine-containing dispersant and multi-functional viscosity modifier that has low chlorine and maximum total sulphated ash not exceeding 1.2 wt %.